High reliability lead-free solder alloy for electronic applications in extreme environments

ABSTRACT

A lead-free solder alloy may comprise tin, silver, copper, bismuth, cobalt, titanium, and antimony. The alloy may further comprise antimony, nickel, or both. The silver may be present in an amount from about 3.1% to 3.8% by weight of the solder. The copper may be present in an amount from about 0.5% to 0.8% by weight of the solder. The bismuth may be present in an amount from about 0.0% (or 1.5%) to about 3.2% by weight of the solder. The cobalt may be present in an amount from about 0.03% to about 1.0% (or 0.05%) by weight of the solder. The titanium may be present in an amount from about 0.005% to about 0.02% by weight of the solder. The antimony may be present in an amount between about 1.0% to about 3.0% by weight of the solder. The balance of the solder is tin.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 62/583,939, filed Nov. 9, 2017, the entire contents ofwhich are incorporated herein by reference.

FIELD

The present disclosure generally relates to lead-free solder alloys forelectronic applications.

BACKGROUND

Solder alloys are widely used for manufacturing and assembly a varietyof electronic devices. Traditionally, solder alloys have been tin-leadbased alloys. Tin-lead based alloys were used to prepare solder withdesired materials properties, including a suitable melting point andpasty range, wetting properties, ductility, and thermal conductivities.However, lead is a highly toxic, environmentally hazardous material thatcan cause a wide range of harmful effects. As a result, research hasfocused on producing lead-free solder alloys with desired materialsproperties.

The present disclosure relates to a high reliability lead-free solderalloy providing a lower undercooling temperature, improvedthermo-mechanical reliability, and high temperature creep resistance inextreme hot and cold weather as compared to certain prior art alloys.

SUMMARY

According to one aspect of the present disclosure, a lead-free alloycomprises: 3.1 to 3.8 wt. % silver, 0.5 to 0.8 wt. % copper; 0.0 to 3.2wt. % bismuth; 0.03 to 1.0 wt. % cobalt; 0.005 to 0.02 wt. % titanium;and balance tin, together with any unavoidable impurities. Optionally,the alloy may further comprise 0.01 to 0.1 wt. % nickel.

According to another aspect of the present disclosure, a lead-free alloycomprises: 3.8 wt. % silver, 0.7 wt. % copper; 1.5 wt. % bismuth; 0.05wt. % cobalt; 0.008 wt. % titanium; and balance tin, together with anyunavoidable impurities. Optionally, the alloy may further comprise 0.05wt. % nickel.

According to another aspect of the present disclosure, a lead-free alloycomprises: 3.1 to 3.8 wt. % silver, 0.5 to 0.8 wt. % copper; 0.0 to 3.2wt. % bismuth; 0.05 to 1.0 wt. % cobalt; 1.0 to 3.0 wt. % antimony;0.005 to 0.02 wt. % titanium; and balance tin, together with anyunavoidable impurities. Optionally, the alloy may further comprise 0.01to 0.1 wt. % nickel.

According to another aspect of the present disclosure, a lead-free alloycomprises: 3.8 wt. % silver, 0.8 wt. % copper; 1.5 wt. % bismuth; 0.05wt. % cobalt; 1.0 wt. % antimony; 0.008 wt. % titanium; and balance tin,together with any unavoidable impurities. Optionally, the alloy mayfurther comprise 0.05 wt. % nickel.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The following is a description of the examples depicted in theaccompanying drawings. The figures are not necessarily to scale, andcertain features and certain views of the figures may be shownexaggerated in scale or in schematic in the interest of clarity orconciseness.

FIG. 1A is a SEM micrograph of a prior art SAC305 alloy in as castcondition.

FIG. 1B is a SEM micrograph of a prior art SAC305 alloy that has beenaged at 125 degree Celsius for 24 hours.

FIG. 2A is a SEM micrograph of an alloy according to the presentdisclosure in as cast condition.

FIG. 2B is a SEM micrograph of an alloy according to the presentdisclosure that has been aged at 125 degree Celsius for 24 hours.

FIG. 3 is a differential scanning calorimetry (DSC) chart for a priorart SAC305 alloy.

FIG. 4 is a differential scanning calorimetry (DSC) chart for an alloyaccording to the present disclosure.

FIG. 5 is a differential scanning calorimetry (DSC) chart for an alloyaccording to the present disclosure.

FIG. 6 is a differential scanning calorimetry (DSC) chart for an alloyaccording to the present disclosure.

FIG. 7 is a differential scanning calorimetry (DSC) chart for an alloyaccording to the present disclosure.

FIG. 8 is a differential scanning calorimetry (DSC) chart for an alloyaccording to the present disclosure.

FIG. 9A is a bar chart showing a comparison between the wetting time oftwo alloys according to the present disclosure and a prior art SAC305alloy.

FIG. 9B is a bar chart showing a comparison between the maximum wettingforce of two alloys according to the present disclosure and a prior artSAC305 alloy.

FIG. 10A is a bar chart showing a comparison between the spread ratio ofan alloy according to the present disclosure and a prior art SAC305alloy.

FIG. 10B is a bar chart showing a comparison between the spreadabilityof an alloy according to the present disclosure and a prior art SAC305alloy.

FIG. 11A is a bar chart showing the spread ratio of an alloy accordingto the present disclosure on three different substrates.

FIG. 11B is a bar chart showing the spreadability of an alloy accordingto the present disclosure on three different substrates.

FIG. 12A is a line chart showing a comparison between the copper wiredissolution rate of an alloy according to the present disclosure and aprior art SAC305 alloy at 260° C.

FIG. 12B is a line chart showing a comparison between the copper wiredissolution rate of an alloy according to the present disclosure and aprior art SAC305 alloy at 280° C.

FIG. 13A shows a comparative series of optical micrographs comparing thecopper wire dissolution rate of an alloy according to the presentdisclosure and a prior art SAC305 alloy at 260° C.

FIG. 13B shows a comparative series of optical micrographs comparing thecopper wire dissolution rate of an alloy according to the presentdisclosure and a prior art SAC305 alloy at 280° C.

FIG. 14A is a bar chart showing a comparison between the hardness of analloy according to the present disclosure and a prior art SAC305 alloy.

FIG. 14B is a bar chart showing a comparison between the hardness of analloy according to the present disclosure and a prior art SAC305 alloy,where both alloys have been isothermally aged at 150° C.

FIG. 15 is a line chart showing stress-strain curves for an alloyaccording to the present disclosure and a prior art SAC305 alloy.

FIG. 16 is a bar chart showing a comparison of the ultimate tensilestrength of an alloy according to the present disclosure and a prior artSAC305 alloy.

FIG. 17 is a line chart showing creep strain as a function of time foran alloy according to the present disclosure and a prior art SAC305alloy both as cast and after aging for 144 hours at 150° C.

FIG. 18A shows a series of micrographs of the interface between an alloyaccording to the present disclosure and an underlying copper substrateafter aging at 150° C. for 240, 720, and 1440 hours.

FIG. 18B shows a series of micrographs of the interface between a priorart SAC305 alloy and an underlying copper substrate after aging at 150°C. for 240, 720, and 1440 hours.

FIG. 19 is a line chart showing total IMC thickness as a function ofaging time at 150° C. for an alloy according to the present disclosureand a prior art SAC305 alloy.

FIG. 20 is a line chart showing Cu₃Sn IMC thickness as a function ofaging time at 150° C. for an alloy according to the present disclosureand a prior art SAC305 alloy.

The foregoing summary, as well as the following detailed description,will be better understood when read in conjunction with the figures. Itshould be understood that the claims are not limited to the arrangementsand instrumentality shown in the figures. Furthermore, the appearanceshown in the figures is one of many ornamental appearances that can beemployed to achieve the stated functions of the apparatus.

DETAILED DESCRIPTION

In the following detailed description, specific details may be set forthin order to provide a thorough understanding of embodiments of thepresent disclosure. However, it will be clear to one skilled in the artwhen disclosed examples may be practiced without some or all of thesespecific details. For the sake of brevity, well-known features orprocesses may not be described in detail. In addition, like or identicalreference numerals may be used to identify common or similar elements.

Novel lead-free solder alloy compositions that are suitable for avariety of electronics applications, particularly in extremeenvironments, are described below. These solder alloy compositions maybe used in various forms. For example, the solder alloy compositions maybe used in the form of a bar, wire, solder powder, solder paste, oranother predetermined preform. These solder alloy compositions are tinbased, in particular tin-silver-copper (sometimes referred to as “SAC”)based.

With the onset of the Internet of Things (IoT), electronic devices arefinding applications in more and more challenging operatingenvironments, leading to higher power densities. As a result, there isan urgent need in the electronic assembly industry for solder that canoperate at higher temperatures. The operating temperature of powerelectronic applications such as automobiles, trains, aerospace, oildrills, downhole gas exploration, and power stations often variesbetween 100° C. and 200° C. Solder joints exposed to elevatedtemperatures for longer times often lose their mechanical strength andstructural integrity.

The addition of a small amount of cobalt to tin-silver-copper soldersignificantly reduces the undercooling temperature and reduces theformation of large Ag₃Sn platelets (the formation of which couldotherwise lead to poor mechanical performance). Further, the synergisticeffect of adding cobalt and titanium results in a refined, uniform, andstable microstructure. Such a microstructure may significantly enhancethe fatigue life of solder joints. As additives to a tin-silver-copperalloy, both bismuth and antimony dissolve in tin matrix and act as solidsolution strengthening agents, which improves the mechanical propertiesand thermo-mechanical reliability of the solder, particularly in harshenvironments.

The compositions shown in Tables 1 to 5 have been found to exhibitdesirable properties that are superior to certain prior art alloys. Forexample, the lead-free solder compositions described in Tables 1 to 5provide lower undercooling temperature, reasonable wetting and spreadingperformance, improved thermo-mechanical reliability, and hightemperature creep resistance in extreme hot and cold weather as comparedto certain prior art alloys.

Table 1 provides several compositions according to the presentdisclosure that comprise tin, silver, copper, bismuth, cobalt, andtitanium. Optionally, these compositions may additionally comprisenickel.

TABLE 1 Composition Composition Composition Composition CompositionRange 1.1 Range 1.2 Range 1.3 Range 1.4 Range 1.5 Element (wt. %) (wt.%) (wt. %) (wt. %) (wt. %) Silver (Ag) 2.0-5.0 3.1-3.8 3.1-3.8 3.1-3.83.1-3.8 Copper (Cu) 0.2-1.2 0.5-0.9 0.5-0.8 0.5-0.9 0.5-0.8 Bismuth (Bi)0.0-5.0 0.0-3.2 1.5-3.8 0.0-3.2 1.5-3.8 Cobalt (Co) 0.0-1.0 0.01-0.2 0.03-0.05 0.03-0.2  0.03-0.05 Titanium (Ti)  0.0-0.02 0.005-0.02 0.005-0.008 0.005-0.02  0.005-0.008 Nickel (Ni) 0.01-0.1  0.01-0.1  Tin(Sn) Balance Balance Balance Balance Balance

Table 2 provides several more compositions according to the presentdisclosure, shown as specific examples.

TABLE 2 Example Example Example Example Example 2.1 2.2 2.3 2.4 2.5Element (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) Silver (Ag) 3.8 3.8 3.83.8 3.8 Copper (Cu) 0.8 0.8 0.8 0.8 0.8 Bismuth (Bi) 1.5 1.5 1.5 3.0 3.0Cobalt (Co) 0.03 0.05 0.05 Titanium (Ti) 0.008 0.008 0.008 Tin (Sn)Balance Balance Balance Balance Balance

Table 3 provides several compositions according to the presentdisclosure that comprise tin, silver, copper, bismuth, cobalt, titanium,and antimony. Optionally, these compositions may additionally comprisenickel.

TABLE 3 Composition Composition Composition Composition CompositionRange 3.1 Range 3.2 Range 3.3 Range 3.4 Range 3.5 Element (wt. %) (wt.%) (wt. %) (wt. %) (wt. %) Silver (Ag) 2.0-5.0 3.1-3.8 3.1-3.8 3.1-3.83.1-3.8 Copper (Cu) 0.2-1.2 0.5-0.9 0.5-0.8 0.5-0.9 0.5-0.8 Bismuth (Bi)0.0-5.0 0.0-3.2 1.5-3.8 0.0-3.2 1.5-3.8 Cobalt (Co) 0.001-1.0  0.03-0.2 0.03-0.05 0.03-0.2  0.03-0.05 Titanium (Ti) 0.005-0.02  0.005-0.02 0.005-0.008 0.005-0.02  0.005-0.008 Antimony (Sb) 0.0-5.0 1.0-3.01.0-3.0 1.0-3.0 1.0-3.0 Nickel (Ni) 0.01-0.1  0.01-0.1  Tin (Sn) BalanceBalance Balance Balance Balance

Table 4 provides several more compositions according to the presentdisclosure, shown as specific examples.

TABLE 4 Exam- Exam- Exam- Exam- Exam- Exam- ple 4.1 ple 4.2 ple 4.3 ple4.4 ple 4.5 ple 4.6 Element (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt.%) Silver (Ag) 3.8 3.8 3.8 3.8 3.8 3.2 Copper (Cu) 0.8 0.8 0.8 0.8 0.80.8 Bismuth (Bi) 1.5 1.5 1.5 3.0 3.0 3.0 Cobalt (Co) 0.03 0.05 0.05 0.05Titanium (Ti) 0.008 0.008 0.008 0.008 Antimony (Sb) 1.0 1.0 1.0 1.5 1.53.0 Tin (Sn) Balance Balance Balance Balance Balance Balance

Table 5 provides several more compositions according to the presentdisclosure, shown as specific examples.

TABLE 5 Example Example Example Example Example 5.1 5.2 5.3 5.4 5.5Element (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) Silver (Ag) 3.8 3.8 3.83.8 3.8 Copper (Cu) 0.7 0.7 0.8 0.8 0.8 Bismuth (Bi) 1.5 1.5 1.5 3.0 3.0Cobalt (Co) 0.03 0.05 0.05 Titanium (Ti) 0.008 0.008 0.008 Antimony 1.01.0 1.5 (Sb) Nickel (Ni) 0.05 0.05 0.05 0.05 0.05 Tin (Sn) BalanceBalance Balance Balance Balance

Controlled additions of bismuth (Bi), antimony (Sb), cobalt (Co), and/ortitanium (Ti) to a tin-silver-copper (Sn—Ag—Cu) system are used torefine the alloy's grain structure and increase the alloy's mechanicalstrength. More specifically, cobalt may be added to the alloy to refinethe grain structure and reduce the undercooling temperature. Further,the synergistic effect of adding cobalt and titanium leads to a refined,uniform, and stable microstructure. Such microstructure significantlyenhances the fatigue life of solder joints. As additives to atin-silver-copper system, bismuth and antimony both dissolve in tin andmay be added to the alloy to provide solid solution strengthening andthus improve the alloy's mechanical properties and any resulting solderjoint's thermal cyclic reliability, particularly in harsh environments.Also, bismuth decreases the solidus temperature of the alloy and reducesits surface tension, thus improving the wettability. Antimony increasesthe mechanical strength of the alloy. Optionally, nickel may be added toimprove further the mechanical properties of the alloy. In addition,elements such as germanium or phosphorus may be added to improve thealloy's oxidation resistance. The proper synergy between the mechanismsdescribed above, which is achieved though the specific compositionranges claimed in the instant application, optimizes the alloy'smechanical properties and any resulting solder joints' resistance tothermal cycles, particularly in harsh environments.

The disclosed composition ranges have been found to exhibit excellentthermal fatigue and creep resistance superior to certain prior artalloys. The high reliability lead-free solder compositions describedhere provide a significant reduction of undercooling temperature,reasonable wetting and spreading performance, improved thermo-mechanicalreliability, and high temperature creep resistance in extreme hot andcold weather. The disclosed solder compositions have been found toexhibit significantly reduced undercooling temperature, and improvedthermo-mechanical reliability and creep resistance. Large Ag₃Snplatelets are prevented from forming. The disclosed solder compositionsare suitable for electronics applications in high temperature or harshenvironments, including but not limited to applications in automobiles,trains, aerospace, oil drills, downhole gas exploration, and powerstations.

FIGS. 1A and 1B show scanning electron microscope (“SEM”) micrographs ofareas of the surface of a prior art alloy comprising 96.5 wt. % tin,3.0% silver, and 0.5 wt. % copper (“SAC305”). FIGS. 2A and 2B show SEMmicrographs of areas of the surface of an alloy according to thecomposition of Example 4.5 shown in Table 4. FIGS. 1A and 2A show thealloys as cast; whereas FIGS. 1B and 2B show the alloys after aging for24 hours at a temperature of 125° C. As can be seen from the SEMmicrographs, the grain structure of the SAC305 alloy (shown in FIGS. 1Aand 1B) coarsens during aging at an elevated temperature. In contrast,the Example 4.5 alloy maintains its finer, more uniform grain structureduring aging at 125° C. (compare FIG. 2A to FIG. 2B). The microstructurecontains Ag₃Sn and Cu₆Sn₅ precipitates, and bismuth and antimony eachdissolve in the tin matrix, which provides solid solution strengthening.Cobalt and titanium act as micro-alloying elements to refine themicrostructure. The finely distributed Ag₃Sn and Cu₆Sn₅ precipitates andsolid solution strengthening stabilize the microstructure during agingat an elevated temperature, particularly in harsh environments.

TABLE 6 Onset of Onset of Pasty Alloy Heating, Cooling, UndercoolingRange, Alloy Composition T₁

 C. T₂

 C. (ΔT = T₁ − T₂)

 C. Prior Art Sn—0.5Cu—3Ag 217 197 20 4 SAC305 Alloy ExampleSn—0.8Cu—3.8Ag—1.5Bi—1.0Sb 217.16 219.85 12.32 2.4 4.1 ExampleSn—0.8Cu—3.8Ag—1.5Bi—1.0Sb—0.03Co—0.008Ti 217.46 208.68 10.31 3.62 4.2Example Sn—0.8Cu—3.8Ag—1.5Bi—1.0Sb—0.05Co—0.008Ti 217.56 213.04 4.523.28 4.3 Example Sn—0.8Cu—3.8Ag—3Bi—1.5Sb 213.73 197.65 16.08 5.92 4.4Example Sn—0.8Cu—3.8Ag—3Bi—1.5Sb—0.05Co—0.008Ti 213.96 206.47 7.49 6.554.5 Example Sn—0.8Cu—3.2Ag—3Bi—3Sb—0.05Co—0.008Ti 215.18 223.51 8.355.67 4.6

As shown in FIGS. 3 to 8, the melting characteristics of solder alloyswere determined by differential scanning calorimetry (“DSC”). Theundercooling (i.e., the temperature difference between the onset ofheating temperature and the onset of cooling temperature) was measuredfor the solder alloys. Undercooling occurs because precipitation ofcrystals is not spontaneous, but requires activation energy. FIG. 3shows the DSC curve for a prior art SAC305 alloy comprising 96.5 wt. %tin, 3.0% silver, and 0.5 wt. % copper. FIGS. 4, 5, 6, 7, and 8 show theDSC curves for alloys according to the compositions of Examples 4.1,4.2, 4.3, 4.4, and 4.5 shown in Table 4, respectively. In addition, datafrom the DSC analysis are shown in Table 6.

High undercooling behaviors of tin-silver-copper (Sn—Ag—Cu) soldersindicate that molten tin solder is difficult to solidify. Highundercooling is attributed to difficulty in nucleating a solid phasefrom the liquid phase. A large undercooling can influencemicrostructural features such tin dendrite, eutectic microstructure,primary intermetallic compounds (Ag₃Sn, Cu₆Sn₅) which in turn affectsthe mechanical properties of the solder. Such undercooling can haveserious impact on the reliability of solder joints and cause anunfavorable situation where joints solidified at different times. Thiscould lead to stress concentration into solidified joint and causemechanical failure. For example, SAC305 alloy has an undercoolingtemperature of 20° C. In contrast, alloys according to the presentdisclosure demonstrate smaller undercooling, for example as low as 4.5°C., as shown for the Example 4.3 alloy.

As can be seen by comparing FIG. 3 with FIGS. 4 to 8, and by reviewingTable 6, several of the example alloys exhibit a noticeable reduction inundercooling as compared to the prior art SAC305 alloy. For example, forthe prior art SAC305 alloy, the onset of heating (T₁) is at 217° C. andthe onset of cooling (T₂) is at 197° C., providing an undercooling (ΔT)of 20° C. For the Example 4.3 alloy, T₁ is at approximately 217.5° C.and T₂ is at approximately 213° C., providing an undercooling (ΔT) ofapproximately 4.5° C.

FIGS. 9A and 9B show a comparison between the wetting time (FIG. 9A) andthe maximum wetting force (FIG. 9B) of a prior art SAC305 alloy, theExample 4.3 alloy, and the Example 4.5 alloy. The wetting experimentswere performed according to IPC (Association Connecting ElectronicsIndustries) standard IPC-TM-650. This standard involves a wettingbalance test that involves determining the total wetting time andmaximum wetting force. A shorter wetting time corresponds to a higherwettability. A shorter wetting time and a higher wetting force reflectsbetter wetting performance and correlates with spread and filletformation under a given soldering process. FIGS. 9A and 9B demonstratethat the wetting properties of the Example 4.3 and 4.5 alloys are betterthan (or at a minimum comparable to) the prior art SAC305 alloy.

Wetting performance of solder can also be expressed in terms of spreadratio and spreadability. The spread area indicates how much solder is onthe soldering pad substrate, and can be indicated as a spread ratio. Aspread test was performed in accordance with the IPC (IPC J-STD-004B, TM2.4.46) and JIS Z 3197 standards. Spread ratio and spreadability wereinvestigated for three different substrates: bare copper (Cu), OrganicSolderability Preservative (OSP) coated copper, and Electroless NickelImmersion Gold (ENIG) plated copper. The solder alloys (circularpreform) were melted onto the substrate being tested using flux. Thewetted area was measured using an optical microscope before and afterthe test. The spread ratio is calculated by wetted area afterreflow/melt divided by wetted area before reflow/melt. The solder heightwas measured to calculate the spreadability (or spread factor).Spreadability was calculated using the following formula, whereS_(R)=spreadability, D=diameter of solder (assumed to be spherical),H=height of spread solder, and V=volume of solder (g/cm³) (estimatedfrom mass and density of tested solder):

$S_{R} = {{\frac{D - H}{D} \times 100\mspace{14mu} {where}\mspace{14mu} D} = {1.248 \times V^{1\text{/}3}}}$

FIG. 10A shows a comparison between the spread ratio of the Example 4.6alloy as compared to a prior art SAC alloy on a bare copper substrate attwo different temperatures (260° C. and 300° C.). FIG. 10B shows acomparison between the spreadability of the Example 4.6 alloy ascompared to a prior art SAC alloy at two different temperatures (260° C.and 300° C.).

FIG. 11A shows a comparison between the spread ratio of the Example 4.6alloy on three different copper substrates (OSP, bare copper, and ENIG)at 255° C. FIG. 11 B shows a comparison between the spreadability of theExample 4.6 alloy on three different copper substrates (OSP, barecopper, and ENIG) at 255° C.

FIGS. 12A, 12B, 13A, and 13B show a comparison between the copperdissolution rate of a prior art SAC305 alloy and the Example 4.3 alloy(Alloy-M) at 260° C. (FIGS. 12A and 13A) and at 280° C. (FIGS. 12B and13B). As can be seen in these figures, the copper dissolution rate isslower for the Example 4.3 alloy as compared to the prior art SAC305alloy. The copper dissolution tests were performed using pure copperwire that was washed, degreased, cleaned in acid solution, rinsed, anddried. The tests were conduct at two temperatures: 260° C. and 280° C.The copper wires were exposed to molten solder for 5 seconds, 10seconds, and 20 seconds. Cross sections of the copper wires wereanalyzed by optical microscopy, including to perform area measurementand analysis.

FIG. 14A shows the hardness values of the Example 4.5 alloy as comparedto a prior art SAC305 alloy. As can be seen from the bar chart, thehardness of the Example 4.5 alloy is approximately twice the hardness ofthe prior art SAC305 alloy. FIG. 14B shows the hardness values of theExample 4.6 alloy as compared to a prior art SAC305 alloy. The Example4.6 alloy retains its hardness after aging, in contrast to the prior artSAC305 alloy, as shown in FIG. 14B, which shows the result of hardnesstesting as cast, after aging 144 hours at 150° C., and after aging 720hours at 150° C.

The coefficient of thermal expansion (CTE) of the alloys according tothe current disclosure was also measured. Mismatches between the CTE ofa solder and an underlying substrate can lead to fatigue failure duringcyclic loading. As the CTE mismatch increases, so too does the shearstrain, which decreases the thermal cycle life of a component. Cracksmay start and propagate at sites of stress concentration due to a CTEmismatch. Cracking in solder joints may be reduced by reducing thedifference between the CTE of a solder and an underlying substrate.Table 7 shows the CTE of an alloy according to the present disclosurecompared to a prior art SAC305 alloy and with reference to the CTE of anexample underlying substrate.

TABLE 7 Alloy/Substrate Temperature CTE, Alloy Composition Range,

 C. ppm/

 C. SAC305 Sn—3.0Ag—0.5Cu 30-150 24.0 ExampleSn—0.8Cu—3.8Ag—3Bi—1.5Sb—0.05Co—0.008Ti 30-150 22.88 4.6 Example Copper30-150 16.7 Substrate

A tensile stress-strain chart of an example alloy according to thepresent disclosure (Example 4.6 alloy) as compared to a prior art SAC305alloy is shown in FIG. 15. Casting solders were machined and cut intorectangular pieces of size 100 mm×6 mm×3 mm. Samples were isothermallyaged at 150 QC for up to 720 hours. Tensile tests were conducted at roomtemperature at a strain rate of 10⁻² s⁻¹. The ultimate tensile strengthand yield strength of the alloys are shown in Table 8. The significantimprovement of tensile strength shown in the Example 4.6 alloy may bedue to the addition of bismuth and solid solution strengthening effect.The Example 4.6 alloy is also shown to be more ductile than the priorart SAC305 alloy. The tensile strength properties of the Example 4.6alloy and prior art SAC305 alloy after aging at 150 QC are shown in FIG.16. Both the Example 4.6 alloy and the prior art SAC305 alloy show areduction in ultimate tensile strength after aging at an elevatedtemperature, but the reduction is considerably more marked for the priorart SAC305 alloy, which exhibited about a 42% reduction in tensilestrength.

TABLE 8 Ultimate Tensile Yield Strength, Strength, Alloy AlloyComposition MPa MPa SAC305 Sn—3.0Ag—0.5Cu 57.72 ± 0.24 49.72 ± 0.19Example Sn—0.8Cu—3.8Ag—3Bi—1.5Sb—0.05Co—0.008Ti 84.11 ± 1.37 79.42 ±1.48 4.6

Creep deformation is a major failure mode of solder joints inmicroelectronic packaging because of the high homologous temperaturesinvolved. Solder experiences thermo-mechanical stresses due to differentcoefficient of thermal expansion (CTE) between the chip and other layerswithin the packages. These stresses can cause plastic deformation over along period of service. Solder alloys may undergo creep deformation evenat room temperature. In real life applications, electronic modules canoperate over a temperature range of −40 QC to +125 QC, which is in therange of 0.48 to 0.87 T_(m) (fraction of the melting temperature of thesolder). For devices under stress, this is a rapid creep deformationrange. Thus, a thorough understanding of creep deformation in lead-freesolder is an important concern for the electronic packaging industry.Casting solders were machined and cut into rectangular pieces of size120 mm×6 mm×3 mm. Samples were isothermally aged at 150 QC for up to 144hours. Creep tests were conducted at room temperature at a stress levelof 10 MPa. As shown in FIG. 17, the Example 4.6 alloy shows superiorcreep resistance as compared to a prior art SAC305 alloy. The creepresistance exhibited by the example alloy may be due to the addition ofmicroalloys to refine the microstructure and strengthening mechanismssuch as solid solution and precipitation hardening.

During a soldering operation, materials from the solid substratedissolve and mix with the solder, allowing intermetallic compounds(IMCs) to form. A thin, continuous, and uniform IMC layer tends to beimportant for good bonding. Without IMCs, the solder/conductor jointtends to be weak because no metallurgical interaction occurs in thebonding. However, a thick IMC layer at the interface may degrade thereliability of the solder joints because a thick IMC layer may bebrittle. IMC layers formed between solder and OSP substrate as afunction of exposure time and temperature were examined. Solder alloyswere melted on an OSP substrate and reflowed in an Electrovert OmniExcel7 Zone Reflow oven using flux. Solder alloy samples were then exposed toan elevated temperature at 150 QC for up to 1440 hours. IMC layers wereevaluated at different periods of aging time.

FIGS. 18A and 18B show a comparison between the IMC layer growth of theExample 4.6 alloy and a SAC305 alloy after aging at 150 QC for up to1440 hours. As can be seen in these figures, both the Example 4.6 alloyand the SAC305 alloy exhibit IMC layer growth. However, the SAC305 alloyshows signs of brittleness, as shown by the presence of Kirkendall voids(for example, after aging for 720 hours). Both alloys show formation ofCu₆Sn₅ and Cu₃Sn layers at the boundary between the solder and thecopper substrate. FIG. 19 shows the total IMC thickness as a function ofaging time. As shown in FIG. 19, the IMC layer for the SAC305 alloy ismuch thicker than for the Example 4.6 alloy. The addition of microalloysto refine the microstructure may limit diffusion, thus also limitingtotal IMC growth. The lower IMC thickness in the Example 4.6 alloylikely makes the Example 4.6 alloy suitable for a longer lifeapplication at elevated temperatures. FIG. 20 shows the total Cu₃Snthickness as a function of aging time. At the interface between Cu₆Sn₅and Cu substrate, a new IMC layer of Cu₃Sn forms for both alloys. In theExample 4.6 alloy, the addition of microalloys suppresses the growth ofCu₃Sn, which may limit the formation of Kirkendall voids.

Some of the elements described herein are identified explicitly as beingoptional, while other elements are not identified in this way. Even ifnot identified as such, it will be noted that, in some embodiments, someof these other elements are not intended to be interpreted as beingnecessary, and would be understood by one skilled in the art as beingoptional.

While the present disclosure has been described with reference tocertain implementations, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedwithout departing from the scope of the present method and/or system. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the present disclosure without departingfrom its scope. For example, systems, blocks, and/or other components ofdisclosed examples may be combined, divided, re-arranged, and/orotherwise modified. Therefore, the present disclosure is not limited tothe particular implementations disclosed. Instead, the presentdisclosure will include all implementations falling within the scope ofthe appended claims, both literally and under the doctrine ofequivalents.

1. A lead-free solder alloy comprising: 3.1 to 3.8 wt. % silver; 0.5 to0.8 wt. % copper; 0.0 to 3.2 wt. % bismuth; 0.03 to 1.0 wt. % cobalt;0.005 to 0.02 wt. % titanium; and balance tin, together with anyunavoidable impurities.
 2. The lead-free solder alloy of claim 1,further comprising 0.01 to 0.1 wt. % nickel.
 3. The lead-free solderalloy of claim 2, comprising 0.05 wt. % nickel.
 4. The lead-free solderalloy of claim 1, comprising 3.8 wt. % silver.
 5. The lead-free solderalloy of claim 1, comprising 0.7 wt. % copper.
 6. The lead-free solderalloy of claim 1, comprising 1.5 to 3.2 wt. % bismuth.
 7. The lead-freesolder alloy of claim 6, comprising 1.5 wt. % bismuth.
 8. The lead-freesolder alloy of claim 6, comprising 3.0 wt. % bismuth.
 9. The lead-freesolder alloy of claim 1, comprising 0.03 to 0.05 wt. % cobalt.
 10. Thelead-free solder alloy of claim 9, comprising 0.05 wt. % cobalt.
 11. Thelead-free solder alloy of claim 1, comprising 0.008 wt. % titanium. 12.A lead-free solder alloy comprising: 3.1 to 3.8 wt. % silver; 0.5 to 0.8wt. % copper; 0.0 to 3.2 wt. % bismuth; 0.05 to 1.0 wt. % cobalt; 1.0 to3.0 wt. % antimony; 0.005 to 0.02 wt. % titanium; and balance tin,together with any unavoidable impurities.
 13. The lead-free solder alloyof claim 12, further comprising 0.01 to 0.1 wt. % nickel.
 14. Thelead-free solder alloy of claim 13, comprising 0.05 wt. % nickel. 15.The lead-free solder alloy of claim 12, comprising 3.8 wt. % silver. 16.The lead-free solder alloy of claim 12, comprising 0.8 wt. % copper. 17.The lead-free solder alloy of claim 12, comprising 1.5 to 3.2 wt. %bismuth.
 18. The lead-free solder alloy of claim 17, comprising 1.5 wt.% bismuth.
 19. The lead-free solder alloy of claim 17, comprising 3.0wt. % bismuth.
 20. The lead-free solder alloy of claim 12, comprising0.05 wt. % cobalt.
 21. The lead-free solder alloy of claim 12,comprising 1.0 wt. % antimony.
 22. The lead-free solder alloy of claim12, comprising 1.5 wt. % antimony.
 23. The lead-free solder alloy ofclaim 12, comprising 0.008 wt. % titanium.